So having spent the last 10 days playing around with various combinations I can confidently answer these questions:
1. Switch off the upstairs radiators via thermostat and same for UFH.
2. Switch one system on via thermostat, does the ASHP start-up? - Thermostat for upstairs - yes. ASHP fires up.
3. Are you getting the correct temps yes or no. - Yes, hot water flows to towel rail and radiators, then switches off once TRV's are happy with room temp (I have now learnt what the numbers on those things correlate to 😃).
4. Switch off everything again
5. Repeat same for other system. - Thermostats for each zone downstairs - yes, ASHP fires up. Continues to for about 10mins when it winds down and the console throws up an H62 error message. Pipework to manifold never gets warm let alone hot.
6. Try both systems what happens. - Thermostats for upstairs and zones downstairs. ASHP fires up. Rads and towel rail get warm. Manifold gets warm water however after a while the H62 error message appears.
7. Bring on the new parts of the system one at a time, what happens? Same as the old/original zones coming on. Flow error and not heating to the manifold.
Pic below is the manifold under the kitchen counter. The actuator was previously on but has been left off today. Doesn;t seem to have made a difference to the issue.
Below the stairs. Apologies for the mess of network cables and disused satellite cables. Could the Honeywell valve in the middle was be cause of the issue?